Degenerative spinal column diseases, for example, disc degenerative diseases (DDD), spinal stenosis, and spondylolisthesis can be corrected by surgical procedures. Typically, spinal decompression is the first surgical procedure that is performed and results in the reduction of pressure in the spinal canal and on nerve roots located therein. Spinal decompression seeks to remove tissue that is applying pressure to the nerve bundle and thus relieve pain. This can result, however, in weakening the spinal column.
Certain surgical procedures, for example posterolateral fusion whereby adjacent vertebral bodies are fused together is often necessary to restore spinal stability following the decompression procedure. Fusion of adjacent vertebral bodies requires that the bone grow together and employs a bone graft or other biological growth agent. In order to maintain the grafting material in place and preserve stability during bone growth, a spinal fixation device is typically used to support the spinal column until a desired level of fusion is achieved. Depending on a patient's particular circumstances and condition, a spinal fixation surgery can sometimes be performed immediately following decompression, without performing the fusion procedure. The fixation surgery is performed in most cases because it provides immediate postoperative stability and, if fusion surgery has also been performed, it provides support of the spine until sufficient fusion and stability has been achieved.
Conventional methods of spinal fixation utilize a rigid spinal fixation device to support and prevent movement of an injured spinal part. These conventional spinal fixation devices include: fixing screws configured to be inserted into the spinal pedicle or sacrum to a predetermined depth and angle, rods or plates configured to be positioned adjacent to the injured spinal part, and coupling elements for connecting and coupling the rods or plates to the fixing screws such that the injured portion of the spin is supported and held in a relatively fixed position by the rods or plates. The connection units prevent further pain and injury to the patient by substantially restraining the movement of the spinal column.
Because the connection units prevent normal movement of the spinal column, after prolonged use, the spinal fixation device can cause ill effects, such as adjacent level syndrome (transitional syndrome) or fusion disease that result in further complications and abnormalities associated with the spinal column. The high rigidity of the rods or plates used in conventional fixation devices causes these disorders due to the patient's joints being fixated by the nature of surgery. The movement of the spinal joints located above or under the operated area is increased. Consequently, such spinal fixation devices cause decreased mobility of the patient and increased stress and instability to the spinal column joints adjacent to the operated area.
It has been reported that excessive rigid spinal fixation is not helpful to the fusion process due to load shielding. As an alternative, semi-rigid spinal fixation devices have been utilized to address this problem while assisting the bone fusion process. For example, U.S. Pat. No. 5,375,823—Navas and U.S. Pat. No. 6,241,730—Alby each disclose a piston configuration mounted between fixing screws having a flexible material or spring element enclosed within a sleeve allowing for axial dampening. Although providing for a greater range of motion than a fixed rod, these devices fail to accommodate for a full range of physiological motion, for example axial torsion or twisting, and are not well-suited for spinal stabilization absent fusion. Thus, in the end these devices do not fully prevent the problem of rigid fixation resulting from fusion.
To solve the above-described problems associated with rigid fixation, semi-rigid and generally flexible devices have been developed. U.S. Publication No. 2006/0264940—Hartmann discloses a flexible spring element connected to a rod and an axially opposed hollow body. The spring element and hollow body have corresponding bores that receive a clamping element. The clamping element has a convex face that abuts the end wall of the internal bore of the spring element during deformation of the spring element under axial loading of the device. The shape of the end of the clamping element controls the spring characteristics of element. While this device functions to provide a greater range of motion during compression it relies upon the spring element as a load bearing structure in tension. This is not an optimal design to handle the long-term cyclical loading the device will experience when implanted.
U.S. Pat. No. 5,672,175—Martin discloses a flexible spinal fixation device which utilizes a flexible rod made of metal alloy and/or a composite material. Additionally, compression or extension springs are coiled around the rod for the purpose of providing de-rotation forces on the vertebrae in a desired direction. However, this approach is primarily concerned with providing a spinal fixation device that permits “relative longitudinal translational sliding movement along [the] vertical axis” of the spine and has a solid construction with a relatively small diameter in order to provide a desired level of flexibility. Because they are typically very thin to provide suitable flexibility, such a rod is prone to mechanical failure and have been known to break after implantation in patients. Similarly, U.S. Publication No. 2007/0270814—Lim shows a vertebral stabilizer that has mobility during compression, extension and rotation. A connecting member such as flexible rods, cables or braided steel are anchored at their distal and proximal ends to engaging portions and are coaxially located within a flexible member. While the connecting members can bend to accommodate shear when the spine is twisted this device has been shown to fail due to fatigue once implanted.
There is no spinal fixation device that can provide for a full range of physiological motion when implanted in a patient. In addition, few devices that attempt to accommodate a range of physiological motion can withstand long-term loading conditions. Therefore, there is a need for an improved dynamic spinal fixation device.